24 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



ments made upon analogous organs is considerable, so that for a compara- 

 tive study they may not be without value . It is therefore of interest to note 

 that the departure in per cent from the average of trichomes on the veins 

 is the same in the long secreting trichome as in the one just considered. 

 Table 7 gives the length of the trichomes, the length and diameters of 

 the heads, and gives the area from which the trichomes were taken. 



TABLE 7. Measurements on short secreting trichomes ofjuglans californica. 



VENTRAL SURFACE. 



Length of 

 trichome. 



Length 

 of head. 



Diameter 

 of head. 



DORSAL SURFACE. 



44.1 

 54.6 

 42.0 

 46.2 



23.1 



27.3 

 25.2 

 25.2 



M 



25.2 

 29.4 



25.2 

 27.3 



The average length of the entire trichome from the ventral surface is 

 39.2 /*, from the dorsal surface is 46.7 M. The average length of the heads of 

 trichomes from the ventral surface is 21 .0 /*, the average diameter is 24.6 p. 

 The averages for the length and the diameter of heads of trichomes taken 

 from the dorsal surface are 25. 2 ^ and 26.7 /*, respectively. 



The measurements given in table 7 are of trichomes from old leaves; 

 those given just previously were from young leaves; we therefore have an 

 additional condition of the trichomes for comparison, /'. e., a comparison 

 of those from the young and from the old leaf. In the instance of the 

 young leaves the average length of the heads of trichomes which were on 

 the veins was found to be greater than the average of those which occur 

 between them. In the old leaves the heads of trichomes were larger on 

 the dorsal surface than on the ventral, although the number was so small 

 that their relation on and between the veins could not well be worked out. 

 A comparison of the average length of heads from the ventral surface of 

 the old leaf and of the young brings out the fact that the heads are longer 



